1. Field of the Invention
This invention relates to methods of reducing the biuret content of urea by reacting an aqueous solution of biuret-containing urea with a strong base to selectively hydrolyze biuret relative to urea, and to urea solutions and solid urea formulations useful in such methods.
Urea is a widely used fertilizer and chemical precursor. Most often it contains some biuret that forms during the manufacturing process or otherwise when urea is heated to 130.degree. C. or above. Biuret can interfere with chemical processing and is toxic to many plants. Biuret phytotoxicity is widely recognized and is regulated or monitored by government agencies and industry. The Indian government prohibits the import of urea containing more than 2 weight percent biuret. The United States agricultural industry generally observes an upper limit of 0.25 weight percent biuret for urea fertilizers classified as "low biuret." This criterion is generally recognized by the citrus and other industries that use urea for foliar fertilization.
Detectable biuret toxicity symptoms have been noted in field tests on lemon and grapefruit in Southern California at biuret levels as low as 0.1 weight percent. Biuret toxicity has also been observed with topically applied urea prills and solutions. Germination inhibition and damage to seedlings has been observed in wheat, barley and similar grain crops at levels of 2 weight percent biuret. These studies, and a comprehensive review of literature available on this subject, are presented by Mithyantha, Kulkarni, Tripathi and Agnihothrudu, Fertilizer News, 1977, pp. 13-18.
Damage to corn has been observed at foliar biuret dosages of 0.2 to 0.5 kilogram per hectare. Thirty percent yield loss was noted in one study at 1.7 kilograms biuret per hectare banded near seeds. Wheat damage has been observed at 0.2 to 0.5 kilogram per hectare foliarly applied with severe toxicity observed at 6.0 kilograms per hectare biuret banded in the soil. Fifteen to twenty ppm soil biuret level has been shown to inhibit barley seed germination while substantial crop damage from foliar application occurs at 0.4 to 0.6 kilogram biuret per hectare.
Similar effects have been observed in rice, citrus, cotton, avocado, beans, soybeans and potatoes, several of which are particularly sensitive to biuret in foliar fertilizers. In citrus, as little as 0.2 kilogram foliarly applied biuret per hectare causes detectable damage. Avocados are damaged by as little as 50 ppm biuret in foliar sprays. As little as 3 kilograms per hectare biuret banded in the soil inhibits potato germination and causes citrus damage in light soils.
In view of these results, it is not surprising that the industry has devoted substantial effort to methods of preventing biuret formation in the first instance, and to methods of reducing its concentration once it is formed. Most present commercial urea plants are capable of producing solid and solution urea containing much less biuret than was previously the case. However, commercial ureas still contain at least 0.5 weight percent biuret almost without exception, and most contain from 1 to 2 weight percent. Biuret content can rise considerably higher if production is not adequately controlled.
2. Description of the Prior Art
One process for removing biuret from urea solutions is described by Fuentes in U.S. Pat. No. 3,903,158, issued Sept. 2, 1975. Fuentes describes a procedure in which a urea solution containing biuret is passed over an ion exchange resin which, according to Fuentes, selectively retains biuret while allowing urea to pass through the exchange column. Although the ion exchange process may be suitable in some applications, it requires specialized apparatus, controls and procedures not readily available or practical in every case. It also converts biuret nitrogen to non-nutrient forms which are lost in the process.
It is, therefore, an object of this invention to provide an improved process for the selective removal of biuret from urea.
It is another object to remove biuret from urea solutions and minimize urea and reactant consumption.
Another object is the production of low biuret urea solutions containing less than 0.25 weight percent biuret based on urea.
Another object is the production of urea solutions containing less than 0.05 weight percent biuret based on urea.
Yet another object is the provision of a method for selectively converting biuret to non-toxic nutrient forms in aqueous urea solutions by base hydrolysis.
Another object is the provision of solid and aqueous compositions for the use in such methods.
Yet another object of this invention is to provide improved urea solutions for agricultural and/or chemical use.
Another object is the provision of solid mixtures of urea and one or more strong bases which, when added to water, will react to selectively eliminate biuret, if present.
Other objects, aspects and advantages of this invention will be apparent to one skilled in the art in view of the following disclosure, the drawing and the appended claims.